SUMMARY:
A keystone pathogen called Porphyromonas gingivalis disarms the immune system to cause inflammation in many forms of gum disease.

Posted: June 18, 2014

When one gets gum disease, their gum tissue becomes red, inflamed and hypersensitive. And to swell the tissue, anaerobic bacteria disarm cells from the immune system in a outnumbered attack, according to a new study. In this fight, no one is rooting for the underdog.

University of Pennsylvania researchers identified the periodontal bacterium Porphyromonas gingivalis as a keystone pathogen, which acts on two molecular pathways to halt immune cells' killing ability and preserve the cells' ability to trigger inflammation. The discovery may open doors for new targets of periodontal disease.

The human body is composed of roughly 10 times more bacterial cells than human cells. In healthy people, there exists an equilibrium between good and bad bacteria. But when disturbances jolt these bacterial populations out of balance, illness such as periodontitis can arise.

The imbalance is called dysbiosis. Although P. gingivalis is fewer in number, they have a disproportional and powerful say in the overall, larger microbial ecosystem. As a result, dysbiosis disables healthy "bystander" gum bacteria while the breakdown products left by inflammation help feed the dysbiotic microbial community, creating a vicious cycle where P. gingivalis thrive.

"Scientists are beginning to suspect that keystone pathogens might be playing a role in irritable bowel disease, colon cancer, and other inflammatory diseases," Dr. George Hajishengallis, lead study author and a professor in the Penn School of Dental Medicine Department of Microbiology, said. "They're bugs that can't mediate the disease on their own; they need other, normally nonpathogenic bacteria to cause the inflammation."

To understand more completely how anaerobic bacteria evade killing without shutting off inflammation, Hajishengallis and his colleagues honed in on neutrophils, which are often the body's first respondents to gum tissue swelling. They used mice to block one of two protein receptors on human neutrophils. In the results, published in Cell Host & Microbe, they discovered that blockading the receptors enhanced the cells' ability to kill the bacteria.

Bacteria buildup
Many people suffer from gum disease (periodontal disease). The condition ranges from simple gum swelling to serious conditions that can result in major damage to the soft tissue and bones that support the teeth. The worst-case scenario is the loss of teeth.

While at the core of the problem there are too many damaging anaerobic bacteria that linger on teeth and in gum pockets, the keystone pathogen P. gingivalis puts into motion a tricky cycle of inflammation that weakens the immune system.

Whether gum disease is stopped, slowed or worsened depends on how the patient cares for his or her teeth. In other words, if you're one of the people diagnosed with periodontal disease, it's up to you to help your immune system by brushing and flossing teeth regularly. It could spell the difference between rotten gums and a clean smile.